This invention relates in general to digital television receivers and in particular to a demodulator for demodulating digital color television signals. In digital video processing systems, the analog video signal is generally digitized, i.e. converted to a digital signal, before signal processing to enable the employment of advanced video features. Commonly a color locked clock signal is used for controlling the analog-to-digital (A/D) converter used to sample the analog video signal. The color-locked clock signal is synchronized or locked to a color burst signal which is a multiple of the color subcarrier frequency Fsc in the National Television Standards Committee (NTSC) system. Another commonly used color system is referred to as the phase alternating line (PAL) system in which a particular relationship between the color or chroma subcarrier frequency and the horizontal scanning frequency is employed. The following equations represent those relationships:
______________________________________ NTSC 4Fsc = 910 .times. Fh, where Fh is the line frequency.
4Fsc = 1135 .times. Fh + 1/2 Fv, where Fv is the frame frequency. ______________________________________
The digital video processing often requires the use of a delay element for delaying the sampled video signal by the equivalent of a horizontal line period. Where the relationship between the color subcarrier frequency and the horizontal or line scanning frequency is fixed, the line delay element may be constructed with an acceptable degree of precision. However, for monochrome video signals and other non-standard video signals, the relationship may not hold and video processing performance may be degraded. The advantage in A/D sampling, when the sampling clock is locked to color, is that the A/D converter samples the video at 90.degree. intervals of the color signal and can therefore simplify the color demodulation function.
In a line-locked system, the line-locked clock signals are fixed to a harmonic of the horizontal sync frequency and the number of clocks in a line period is constant, irrespective of the horizontal scan frequency. In a line-locked system, the sampling clock of the A/D converter is locked to the line (Fh) and the accuracy of the delay element is independent of the line scan frequency. This yields a significant advantage when the system encounters signals with a non-standard relationship between the color subcarrier and horizontal scan frequency. The major difficulty in the line-locked color sampling system is that the horizontal frequency Fh may not bear the proper relationship to the color subcarrier frequency Fsc as described by the NTSC and PAL transmission standards. A simple color locked A/D converter for the color demodulation function may not operate properly with non-standard video signals. In particular, if the horizontal frequency differs from that established for either the NTSC or PAL system, a phase error .theta..sub.e, which in the color-locked illustration is a constant, will continuously vary over the entire horizontal line. Thus a system for using a line-locked clock for a chroma A/D converter will require a color demodulator for continuously correcting each pair of U and V color component data for the continuously varying phase error .theta..sub.e.
The present invention discloses a practical color demodulation system for a digitized line-locked color signal.